Download Lecture 5 - Crystallization

Mineral assembly
• Most minerals will deal with ionic bonds
between cations and anions (or anionic
subunits which are themselves mostly
covalent but do not dissociate)
• Assembly of minerals can be viewed as
the assembly of individual ions/subunits
into a repeatable framework
• This repeatable framework is a crystal or
crystalline material
Mineral Assembly
• Isotropic – same properties in every
direction
• Anisotropic- different properties in different
directions  most minerals are this type
• Assembly of ions from melts, water, or
replacement reactions which form bonds
• The matrices the ions are in always contain
many different ions – different conditions of
formation for the same mineral creates
differences…
Polymorphs
• Two minerals with the same chemical formula but
different chemical structures
• What can cause these transitions??
•sphalerite-wurtzite
•pyrite-marcasite
•calcite-aragonite
•Quartz forms (10)
•diamond-graphite
Complexes  Minerals
• Metals in solution are coordinated with ligands
(Such as H2O, Cl-, etc.)
• Formation of a sulfide mineral requires direct
bonding between metals and sulfide
– requires displacement of these ligands and
deprotonation of the sulfide
• Cluster development is the result of these
requirements
Mineral growth
• Ions come together in a crystal – charge is
balanced across the whole
• How do we get large crystals??
– Different mechanisms for the growth of
particular minerals
– All a balance of kinetics (how fast) and
thermodynamics (most stable)
Nucleation
• Aggregation of molecules builds larger and
larger molecules – becomes a nucleus at
some point
• Nucleus – size of this is either big enough
to continue growth or will re-dissolve
(Critical Size)
• Overall rate of nucleus formation vs.
crystal growth determines crystal
size/distribution
Crystal Shapes
• Shape is determined by atomic
arrangements
• Some directions grow faster than others
• Morphology can be distinct for the
conditions and speed of mineral
nucleation/growth (and growth along specific
axes)
Ostwald Ripening
Larger crystals are more stable than smaller crystals
– the energy of a system will naturally trend towards
the formation of larger crystals at the expense of
smaller ones
In a sense, the smaller crystals are ‘feeding’ the
larger ones through a series of dissolution and
precipitation reactions
Figure 3-17. “Ostwald ripening” in a monomineralic material. Grain boundaries with significant
negative curvature (concave inward) migrate toward their center of curvature, thus eliminating
smaller grains and establishing a uniformly coarse-grained equilibrium texture with 120o grain
intersections (polygonal mosaic). © John Winter and Prentice Hall
Small crystals…
• In the absence of ripening, get a lot of very
small crystals forming and no larger
crystals.
• This results in a more massive
arrangement
• Microcrystalline examples (Chert)
• Massive deposits (common in ore
deposits)
Topotactic Alignment
•Alignment of smaller grains in space – due to magnetic
attraction, alignment due to biological activity (some microbes
make a compass with certain minerals), or chemical/
structural alignment – aka oriented attachment
Igneous Textures
Figure 3-1. Idealized rates of
crystal nucleation and growth as
a function of temperature below
the melting point. Slow cooling
results in only minor
undercooling (Ta), so that rapid
growth and slow nucleation
produce fewer coarse-grained
crystals. Rapid cooling permits
more undercooling (Tb), so that
slower growth and rapid
nucleation produce many finegrained crystals. Very rapid
cooling involves little if any
nucleation or growth (Tc)
producing a glass.